Swiss Pat. No. 440,134 describes a double-wall tank having an inner wall, for example made of steel, and an outer wall made of plastic. A spacer structure in form of a wire grid or wire mesh or wire weave is located between the inner wall and the outer wall. The wire mesh or grid forms a substrate for a foil forming the outer wall, located on the wire grid or wire weave. The foil described may be a cellulose foil of about 0.06 mm thickness, or an aluminum foil of about 0.1 mm thickness. The foils are used as substrates for a relatively thick layer made of glass fiber reinforced plastic, thereby forming a double-wall tank. The double wall is used to increase the safety and reliability of the tank with respect to leaks of liquid retained in the tank. A leakage warning apparatus is provided which indicates a leak at the inner wall.
The construction is difficult to make since the wire mesh as well as the foil must be handled and placed. The foil, effectively, adds only little to the strength of the wall. The wall, thus, must be made of a relatively thick plastic layer which, additionally, is reinforced with fibers, typically glass fibers. This plastic layer requires much material, and the construction thereof is relatively complex since it is applied by spraying. The spray apparatus must include a glass fiber cutter. It is unavoidable that glass fibers become loose and fly about. The glass fibers easily penetrate the skin of operators and result in itching. Loose glass fibers, additionally, interfere with working within the tank, and, in some cases, make working within the tank completely impossible.
The Swiss Pat. No. 440,134 describes such a tank in which only the outer wall is made of such plastic material. The outer wall must be carefully made to avoid the inclusion of air bubbles, requiring rolling out of the applied plastic material to eliminate the formation of any such air bubbles. Air bubbles may, otherwise, be introduced upon spraying of the plastic material together with the glass fibers. The rolling-out of the fiber-reinforced plastic wall is time-consuming, and hence expensive.
Swiss Patent of Addition No. 466,498, forming an addition to the previously discussed main patent. Swiss No. 440,134, discloses use of a plastic grid or mesh instead of a wire grid or mesh. The plastic grid has the advantage that it is substantially more flexible than metal, and is also somewhat stretchable, so that its application is simplified. In all other respects, however, the difficulties of manufacture which arise in the construction explained in connection with the main patent still pertain.
Swiss Pat. No. 471,728 describes a double-wall tank having an inner wall made of steel and an outer wall made of plastic. A spacer in the form of a foil is described, the spacer being formed with impressed or punched bumps or warts, which are facing the inner wall and are supported against the inner wall. The description proposes a spacer wall of about 0.2 mm sheet steel or sheet aluminum.
The arrangement has an advantage over the tank in accordance with the previously discussed Swiss Pat. No. 440,134 since this very thin foil can be placed about the surface of the inner wall more easily than the wire mesh which, when using a wire diameter sufficient to form a suitable chamber or hollow space between the tank walls, is comparatively stiff. All other disadvantages of the tank described in connection with Swiss Pat. No. 440,134, however, still pertain. The description is directed to the manufacture of an outer wall made of plastic, but not to an inner wall made of plastic.
Swiss Pat. No. 480,243 describes an inner jacket for a storage container which is liquid-tight. An embossed foil is located in the region of the vertical side walls. The bottom of the tank utilizes as an intermediate layer a porous mass, for example porous concrete.
Swiss Pat. No. 517,630 describes a tank to retain liquid having non-metallic walls. This liquid-containing tank has electrical conductors located on one of the wall surfaces so that the freedom from pores of the double walls of the tank can be tested; any pores which might occur can be localized by means of a spark inductor. The outer wall of the container, preferably, is made of concrete.
The conductive material is, preferably, an aluminum foil formed with bumps or warts or longitudinal ribs or ridges, for example in the form of creases. The aluminum foil may have from between 0.02 to 0.1 mm thickness. The bumps, warts, or ribs or ridges, for example longitudinal creases, form a test space for leakage between an outer wall and an inner wall. The inner wall is formed by a glass fiber reinforced plastic, for example polyester, polyurethane, or a thermoplastic material which has the required stability, pressure resistance and corrosion resistance with respect to the liquid to be introduced into the tank. The relatively thick and stiff plastic layer, and not the metallic layer, provides the necessary strength for the inner wall.
Swiss Pat. No. 618,658 describes a similar construction which is directed also to form a double bottom of an upright tank. Rather than using an aluminum foil with bumps or longitudinal creases, the description states that strong paper, such as Kraft paper or the like, or heavy wrapping paper may be used, formed with projections and recesses.
The paper is made by first applying a coating ink thereon which includes glass fibers having a fiber length of between 2.5 mm to 5 mm. The first layer applied is dried for 24 hours and then a second, usually colorless layer is applied. The strength of the inner wall is primarily governed by the layer reinforced with glass fibers; the heavy paper, formed with projections and depressions, hardly contributes anything to the strength. It is clear that, for such a construction, the plastic layer must be relatively thick. Consequently, the manufacture is time-consuming, particularly since after application of the first layer, a period 24 hours must be permitted to elapse. The use of plastic to manufacture a thick plastic layer is extensive.
U.S. Pat. No. 4,739,895, reissued under U.S. Pat. No. Re. 33,421, by the inventor hereof, describes a tank having a double bottom. The double bottom is formed by a bottom layer and a top layer, in which the top layer is constructed of a plurality of foil or sheet elements, each of a material having a strength characteristic of and a thickness requisite for the top layer of the double bottom, that is, to support the contents of the tank above the chamber defined by the two layers. The plurality of foil or sheet elements are located next to each other, preferably with overlap. The foil or sheet elements are formed with projections extending from a major surface of the foil or sheet element towards the bottom of the layer to have a puckered configuration. A fiber reinforced resin layer is applied and bridges over the terminal edges of adjacent foil or sheet elements. The foil or sheet elements, with the puckered surface facing downwardly towards the bottom of the tank then is covered-over in its entirety by a protective plastic layer cover. This protective layer cover will extend over the foil and sheet elements as well as over the fiber reinforced resin layer bridging the respective foil or sheet elements and connecting them together. It is of a thickness just sufficient for protection of the foil or sheet layer elements against attack from the contents of the tank without, however, substantially contributing to the strength of the top layer. Thus, the material of the protective layer prevents, for example, chemical attack on the puckered or embossed foil and fiber reinforced bridging elements which join the respective foil units to form a complete tank bottom.
This tank construction has a substantial advantage because, in contrast to the heretofore described structures, the embossed or puckered or ribbed foil is not merely an underlay for a plastic layer which forms the actual inner wall; rather, the embossed or puckered foils or sheet elements themselves provide the necessary strength for the inner wall. The plastic layer is, essentially, merely a protective layer to protect the foil or sheet elements with respect to the contents of the tank. This inner wall requires only little plastic material for its manufacture, since, for the major portion of the inner wall, the plastic layer need have a thickness of only about 1 mm to provide protection. Only at the junctions between individual puckered foil elements or sheets will the inner wall be somewhat thicker, and also be reinforced with a glass fiber reinforcement to form the junctions between the foil elements, so that the inner wall, there, will have the necessary strength.
This earlier patent by the inventor hereof also describes a method to make a double-wall tank. Preferably, but not necessarily, the inner side of the outer wall is coated with a protective layer of plastic material. The embossed, puckered or ribbed foil or sheet elements are then laid on the inner wall. Preferably, the placement is such that the individual sheet elements overlap. Plastic adhesive material is precoated at the end or edge surfaces of the sheet element, and, while still liquid, a glass fiber tape is laid thereon and rolled into the plastic mass.
When the overlapped foils are joined, a space will be formed between the inner wall and the outer wall to permit checking of the tank for leakage. The entire surface, now covered with the embossed, connected foil elements is then coated with a thin plastic coating, for example an epoxy. After all the plastic materials have cured or set, a tightness or leakage test or test for pores is made by a spark detector.
This tank was a substantial advance, both with respect to structure as well as cost of manufacture, in relation to prior art tanks. It has been found, however, that under highly stressful conditions, deformation of the inner wall may result which caused fissures to occur in the plastic coating, resulting in leakages.
German A 37 39 453, Panzer, describes a multi-layer internal structure to permit tests for leakage, for subterranean or buried tanks. The inner structural layer has at least six individual layers, namely an outer one formed of concrete, a porous or permeable drainage layer, an outer sealing layer, a permeable textile intermediate layer, and a plurality of layers of epoxy resin with fiberglass inserts for reinforcement, Such constructions are not suitable for much smaller tanks, and are extremely expensive to make. To prepare each layer, it is necessary to apply epoxy resin, to place a layer of glass fibers thereon which, then, is well rolled into the still liquid epoxy resin by means of a roller. Some time must then be permitted to elapse to permit the layer to cure. Before a subsequent layer can be made, the preceding layer must have cured, which substantially increases the manufacturing time and causes long delays and interruption in the smooth work flow. This has been described above already with reference to Swiss Patent 618,658, Grossenbacher. This process, thus, is not only very time-consuming and expensive but has the additional disadvantage that the layers do not adhere well above each other and have a tendency to delaminate.